While silica is a widely preferred sorbent due to its high surface area and sorption energy, certain applications requiring highly controlled selective sorption are hindered by its relatively high and nonuniform energy for adsorption as well as its nonspecific interaction with many materials. An example of a selective sorption process hindered by the nonuniform, high surface energy of silica is the chromatography of pharmaceuticals, proteins, peptides, nucleotides and other hydrophilic substances important in biotechnology. To overcome this problem, there have been many methods developed for the modification of silica to reduce and make uniform its adsorptive properties and to improve its selectivity for one or more compound(s) of interest.
Early work on the modification of silica described (1) the use of an esterification reaction between surface silanol groups and an alcohol to give Si-O-C linkages (Halasz and Sebestian, Angew. Chem. (Int. Ed.) 8 453 (1969); Deuel et al., Helv. Chim. Acta 119:1160 (1959)), (2) reactions of alkylchlorosilanes with surface silanols to form Si-O-Si-C linkages (Hunter et al., Indus. and Engin. Chem 39:1389 (1947)), (3) chlorination of the silanols followed by reaction with an amine to form Si-N bonds (Deuel et al., supra), and (4) chlorination of a silicon dioxide surface followed by alkylation (Wartmann and Deuel, Helv. Chim. Acta 119:1166 (1959)).
Other work relating to modification of silica is described in several patents and publications. U.S. Pat. No. 3,359,214 to Aftandilian, for example, describes a method of increasing the number of surface hydroxyl groups on silica, by reaction of the surface silanols first with silicon tetrachloride and following with hydrolysis of the silicon-chlorine bonds so formed. U.S. Pat. No. 3,839,385 to Meiller shows the "grafting" of organic groups (typically halogenated organosilanes having a central ester linkage) onto silica gel particles. Several patents show the introduction of sulfonate or sulfonic acid groups into a silica matrix, e.g. U.S. Pat. Nos. 4,257,916 to Hancock et al. and 4,661,248 to Ramsden et al. U.S. Pat. No. 4,520,122 to Arena discloses preparation of metal-alkyl complexes on the surface of silica particles.
Thus, a number of chemically modified silica particles are commercially available for a variety of chromatographic purposes. An important consideration in the preparation of such modified particles is their stability under a range of conditions of use, e.g. with regard to variation in temperature, pH and the like. The original commercially modified silicas based on the Si-O-C linkages are thermally stable and thus suitable for gas chromatography, which is an anhydrous method. However, such silicas are readily hydrolyzed and are thus unsuitable for liquid chromatography of pharmaceuticals, biomolecules, and the like, which requires the use of aqueous eluting solvents. The development of the Si-O-Si-R linkages (in U.S Pat. No. 3,956,179 to Sebestian et al., for example, R.dbd.C.sub.18 H.sub.32), which linkages are hydrolytically stable to neutral and weakly acidic eluants, extended the application of chemically modified silicas into liquid chromatography. However, the Si-O-Si-R linkages, being unstable to basic hydrolysis, cannot be used with alkaline eluants, which is a serious limitation. Additionally, stability of Si-O-Si-R, where R is less than five carbon atoms, is limited even in mildly alkaline eluants due to reduction of hydrolytic protection afforded by the larger hydrophobic R groups (e.g., C.sub.18 H.sub.32). Because alkaline eluants and chemically modified silica particles incorporating short hydrocarbon chains (i.e., R is CH.sub.3 or C.sub.2 H.sub.5) are preferred for analysis of proteins and peptides, currently available chromatography particles based on silica are inadequate for many applications.
Attempts at using a chlorination/Grignard or chlorination/organolithium reaction sequence to modify silica sorbents have been limited by two factors. The first is the relative ease of producing materials from the one-step alkylchlorosilane reactions (e.g., as described in U.S. Pat. No. 3,956,179 to Sebestian et al., cited supra) versus the two-step chlorination/Grignard process. For alkylchlorosilation, refluxing the reagent in a suitable solvent (such as toluene) in the presence of silica yields the desired product. The second limitation involves the residual salts (MgBrCl or LiCl) which are deposited on the surface after the second reaction with the Grignard or organolithium compound. Both of these problems have prevented further development of this two-step reaction sequence.
In spite of these problems, it is nevertheless desirable to develop a process utilizing such a two-step reaction sequence in order to increase the versatility of the modification process and the stability of the final product, i.e., to variation in temperature and pH as well as to hydrolysis generally. That is, in other types of modification methods, it is frequently not possible to synthesize the appropriate silane material which will then yield the desired final product. In this case, synthesis of a Grignard reagent or organolithium compound offers an alternative to the alkylchlorosilane pathway and more versatility insofar as the number and type of organic substituents which may be introduced onto the silica surface. Such an alternate method is only useful, however, if the undesirable salt by-products can be eliminated and if the reaction proceeds in high yield, i.e., comparable to that of the alkylchlorosilane reaction. Thus, while the chemistry for creating Si-C linkages has been known, practical applications of this chemistry to the production of chemically modified sorbent particles containing a Si-C bond has never been achieved due to technical problems in synthesis and purification.